I have trouble getting a standard rate of infection of THP-1 cells with the intracellular pathogen Leishmania. Does anyone know how best to standardize this infection?

You may check this paper at the following link:
http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=12662300&cmd=showdetailview&indexed=google
1: Scand J Immunol. 2003 Apr;57(4):366-74. Links
Infection of human mononuclear phagocytes and macrophage-like THP1 cells with Leishmania donovani results in modulation of expression of a subset of chemokines and a chemokine receptor.Dasgupta B, Roychoudhury K, Ganguly S, Akbar MA, Das P, Roy S.
Department of Immunology, Indian Institute of Chemical Biology, Calcutta 700-032, West Bengal, India.

The expression of chemokines and chemokine receptors was studied in Leishmania donovani (LD)-infected human mononuclear phagocytes and the human monocytic cell line THP1. Our studies showed that LD infection caused the upregulation of three beta chemokines (macrophage inflammatory protein-1 alpha (MIP-1alpha), MIP-1beta and RANTES (regulated on activation normal T cell expressed and secreted)), one alpha chemokine (interleukin-8 (IL-8)) and the CC chemokine receptor 5 (CCR5) but not CCR1, as evident from reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. The CCR5 upregulation in human mononuclear phagocytes and THP1 cells was also evident by confocal microscopy. The possible association of such upregulation in relation to Leishmania and human immunodeficiency virus (HIV) coinfection was discussed.